The present invention relates to a valve timing control system for an internal combustion engine, which performs variable control of opening and closing timing of an intake or exhaust engine valve in accordance with the engine operating conditions.
Typically, the valve timing control system controls opening and closing timing of an engine valve by controlling the phase of rotation of a crankshaft and a camshaft on a power transfer path from the crankshaft to the camshaft. Specifically, the system comprises a driving rotator coupled to the crankshaft through a timing chain and the like, a follower rotator coupled to the camshaft and to which the driving rotator is mounted to enable relative rotation as required, and a mounting-angle control mechanism interposed between the two rotators to control a mounting angle formed therebetween. Operating-force providing means provide an operating force to the mounting-angle control mechanism when required to change the phase of rotation of the crankshaft and the camshaft.
As for the operating-force providing means which include a hydraulic mechanism typically, various electromagnetic mechanisms have been developed in recent years. Some valve timing control systems using an electromagnetic force in the operating-force providing means include an electric motor unit between the driving rotator and the follower rotator. However, since an electromagnetic coil of the motor unit should integrally be mounted to one of the driving rotator and the follower rotator, the systems need a slip ring having insecure durability for energization of the coil, and are susceptible to torque variation due to increased inertia force of the rotators.
JP-A 10-103114 discloses a valve timing control system which is free of such inconvenience, wherein an electromagnetic coil is fixed to a casing non-rotatably mounted to an engine block so as to make a magnetic field or driving force produced by the coil act on a mounting-angle control mechanism through an air gap.
With the valve timing control system disclosed in the reference, however, the driving rotator and the follower rotator (particularly, the latter) are axially displaced together with the camshaft in accordance with engine operation, while the electromagnetic coil is fully fixed to the engine block through the casing, so that a driving force resulting from the coil is not stabilized during engine operation, often causing unstable control of valve timing. Specifically, the coil provides through the air gap a driving force to the mounting-angle control mechanism, which is mounted, together with the driving rotator and the flower rotator, to the camshaft to enable unitary axial displacement. Thus, when the camshaft is axially displaced in accordance with engine operation, the air gap varies with that displacement, leading to unstable driving force resulting from the electromagnetic coil.
It is, therefore, an object of the present invention to provide a valve timing control system for an internal combustion engine, which always allows desired control of valve timing regardless of axial displacement of the driving rotator and the follower rotator.
The present invention provides generally a system for controlling a valve timing in an internal combustion engine, which comprises: a driving rotator rotated by a crankshaft of the engine; a follower rotator provided to a camshaft of the engine, the follower rotator receiving power from the driving rotator; a stationary member; and an electromagnetic coil mounted to the stationary member, the electromagnetic coil producing a magnetic field to control an angle formed between the driving rotator and the follower rotator, the electromagnetic coil having rotation restricted and axial displacement allowed by the stationary member, the electromagnetic coil being engaged with one of the driving rotator and the follower rotator to enable rotation with respect to the one and axial displacement together therewith.